Separation of compounds having close boiling points usually requires a more complicated process, relative to conventional distillation methods. Extractive distillation is one technique developed for this purpose, which has been applied commercially and is becoming increasingly important as a separation method, particularly in the refining and petrochemical industries. A common characteristic of extractive distillation methods is the use of a heavy solvent, i.e., a high boiling point, extractive agent that is added to the mixture of the compounds to be separated. This extractive agent functions to increase the relative volatility of these compounds. Relative volatility provides an indication of the degree of their separability (ease of separation) by distillation from a liquid mixture of these compounds. In addition to altering the relative volatility, an effective extractive agent should also itself be easily separated from the distillation products. Thus, a significant difference in boiling point between the extractive agent and the compounds to be separated is desirable. The extractive agent plays an important role in the design of extractive distillation processes. Therefore, the selection of a suitable extractive agent is essential to ensuring the effective and economical design of an extractive distillation process.
Ethylbenzene is a hydrocarbon compound with high commercial utilization and value. A major use is for the production of styrene, which in turn serves as an intermediate in the manufacture of polystyrene. Ethylbenzene can be synthesized from the reaction of benzene and ethylene, or alternatively it can be recovered from an impure hydrocarbon mixture. For example, impure mixtures containing ethylbenzene are commonly produced as by-products of various petrochemical processes. Impure hydrocarbon mixtures containing ethylbenzene normally also contain other hydrocarbons, most notably other C8 aromatic hydrocarbons (e.g., xylenes) having boiling points close to that of ethylbenzene.
Examples of impure ethylbenzene-containing mixtures are obtained from naphtha cracker effluents, in which pyrolysis gasoline streams are generated. It is desirable to extract valuable aromatic compounds, and mixtures of compounds, including benzene, toluene, ethylbenzene, and xylene from pyrolysis gasoline for further utilization. Benzene and toluene can be conveniently separated with known techniques, leaving a mixed C8 hydrocarbon stream, comprising ethylbenzene in combination with xylenes. Such mixtures typically contain more than 60 wt-% ethylbenzene (“high ethylbenzene”), with most or all of the remainder being mixed xylenes that co-boil with each other and with ethylbenzene, rendering these components difficult to separate by conventional distillation.
Other impure ethylbenzene-containing mixtures are those used in processes for the production of para-xylene, in which a mixed C8 hydrocarbon stream is separated from such mixture, for example a reformate stream that is a feed to a xylene column. This mixed C8 hydrocarbon stream is then further separated, for example using known simulated moving bed processes employing a suitable adsorbent, into a para-xylene rich stream (e.g., recovered as a para-xylene rich product) and a para-xylene lean stream (or para-xylene depleted effluent). The para-xylene lean stream generally comprises meta-xylene, ortho-xylene, and minor amount of ethylbenzene, generally not exceeding 20 wt-% (“low ethylbenzene”). In order to increase the yield of para-xylene, the para-xylene lean stream is subsequently processed in a catalytic conversion step to dealkylate ethylbenzene. The dealkylated products are separated from the effluent obtained from this step, and the remaining stream, substantially comprising meta-xylene and ortho-xylene, is isomerized to increase the overall yield of the desired para-xylene isomer. This para-xylene enriched isomerate (or xylene-equilibrated isomerate, to the extent that para-xylene concentration is increased in the direction of its equilibrium concentration with the other xylene isomers) is then generally recycled to the xylene column.
Various attempts have been made to facilitate the separation of hydrocarbons from impure mixtures. Examples of these are described in GB 1,198,592; U.S. Pat. Nos. 3,105,017; 5,397,441; 4,299,668; WO 2016/036326; WO 2016/036388; and WO 2016/036392, which disclose the use of various operating conditions and/or extractive agents for this purpose. However, the proposed solutions generally involve one or more of high energy consumption, complex process flow schemes, the use of toxic agents, and/or other parameters that are generally undesirable. Accordingly, there remains a need in the art for extractive distillation processes that overcome various drawbacks associated with conventional techniques.